Polyarylene sulfide (hereinafter, also referred to as “PAS”), represented by polyphenylene sulfide (hereinafter, also referred to as “PPS”), is an engineering plastic having excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability, and the like. PAS has been widely used in a wide variety of fields, such as electric/electronic devices and devices for automobiles, since PAS can be formed into various molded products, films, sheets, fibers, and the like by ordinary melt processing methods, such as extrusion molding, injection molding, and compression molding.
As a representative method of producing PAS, a method in which a sulfur source and a dihalo aromatic compound, such as p-dichlorobenzene (hereinafter, also referred to as “pDCB”), are subjected to a polymerization reaction in an organic amide solvent, such as N-methyl-2-pyrrolidone (hereinafter, also referred to as “NMP”) has been known. Furthermore, a method of producing PAS in which a sulfur source and a dihalo aromatic compound are subjected to a polymerization reaction in an organic amide solvent in the presence of an alkali metal hydroxide has been known (Patent Documents 1 and 2 and the like). Although an alkali metal sulfide is mainly used as the sulfur source, a method in which an alkali metal sulfide formed by reacting an alkali metal hydrosulfide and an alkali metal hydroxide in situ is used as a sulfur source is also known.
For a method of producing PAS in which a sulfur source and a dihalo aromatic compound are subjected to a polymerization reaction in an organic amide solvent in the presence of an alkali metal hydroxide, an attempt has been made to set conditions to stably perform the polymerization reaction and to suppress side reactions. That is, in this polymerization method, since an alkali metal hydroxide, such as sodium hydroxide (NaOH), is used at a high concentration during the polymerization reaction, the organic amide solvent is readily decomposed by the alkali metal hydroxide and the decomposition product may be the cause of side reactions.
For example, when NMP and NaOH are reacted, the ring of the NMP is opened to form sodium methyl amino butanoate ((CH3)NH—CH2—CH2—CH2—COONa; hereinafter, also referred to as “SMAB”). This compound reacts with p-dichlorobenzene, which is a monomer, to form chlorophenyl methyl amino butanoic acid (hereinafter, also referred to as “CPMABA”).
CPMABA and/or SMAB participates in the polymerization reaction, and forms a methyl amino butanoic acid group at a terminal of the produced PAS, thereby acting as a polymerization terminator of the PAS, and thus it is difficult to obtain highly polymerized PAS. Furthermore, since CPMABA is a byproduct formed by reacting SMAB with p-dichlorobenzene, which is a monomer forming PAS, the formation of CPMABA results in loss of raw materials that form PAS and reduction in yield of PAS.
Furthermore, in cases where the obtained PAS contains a compound having a nitrogen atom as an impurity, when the obtained PAS is injection-molded or extrusion-molded, a problem occurs in that the impurity adheres to the mold or die. Since such contamination of the mold, die, or the like due to a compound containing a nitrogen atom negatively affects the quality of molded product, frequent cleaning is required.
Patent Document 3 discloses a method of producing PAS, in which a sulfur source containing an alkali metal hydrosulfide and a dihalo aromatic compound is subjected to a polymerization reaction in an organic amide solvent and, from the initiation to the completion of the polymerization reaction, pH of the polymerization reaction mixture is controlled to be in a range of 7 to 12.5 by adding an alkali metal hydroxide continuously or in portions to the polymerization reaction mixture. According to this method, PAS with a high purity and high melt viscosity can be stably obtained by suppressing side reactions and decomposition reactions. However, since the pH of the polymerization reaction mixture needs to be controlled so that the pH is in a range of 7 to 12.5 from the initiation to the completion of the polymerization reaction, apparatus for continuous addition and special controlling system under high temperature and high pressure are required.
Furthermore, Patent Document 4 discloses a method of producing PAS, including: a preparation step of preparing a preparation mixture containing an organic amide solvent, a sulfur source, an alkali metal hydroxide, water, and a dihalo aromatic compound, and having a pH of 12.5 or higher; a first-stage polymerization step of heating the preparation mixture to a temperature of 170 to 270° C. to initiate a polymerization reaction, thereby forming a prepolymer having a dihalo aromatic compound conversion rate of 50% or greater; and a second-stage polymerization step of adding a phase separation agent into the reaction system containing the prepolymer and adding an alkali metal hydroxide in an amount corresponding to 1 to 10 mol % per 1 mol of the sulfur source (charged sulfur source) at a time or in portions, thereby continuing the polymerization reaction at a temperature of 240 to 290° C. This method is a method of producing PAS that stably and efficiently produces PAS having excellent reactivity with a silane coupling agent and is not yet satisfactory from the perspectives of polymer yield and byproduct formation.
Therefore, as a method of producing PAS in which a sulfur source and a dihalo aromatic compound are polymerized in an organic amide solvent, a method of producing PAS that can suppress side reactions and produce PAS with a high purity and a high molecular weight at a high yield has been desired.